- Email: [email protected]
Ag superconducting composites
Physica C 471 (2011) 395–399
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Near-oscillatory relaxation behavior of levitation force in infiltration and growth processed bulk YBCO/Ag superconducting composites R. Parthasarathy, M.M. Lakshmi, V. Seshubai ⇑ School of Physics, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e
i n f o
Article history: Received 25 November 2010 Received in revised form 14 March 2011 Accepted 7 April 2011 Available online 13 April 2011 Keywords: Relaxation of levitation force YBCO/Ag superconducting composites Bistable equilibrium model Current structure in superconductor
a b s t r a c t Time relaxation behavior of levitation force has been studied in IGP bulk YBCO/Ag superconductor using levitation force measurements as these measurements throw light on the magnetic relaxation in superconductors and the underlying vortex dynamics, pinning mechanisms and the nature of pinning forces. The measurements have revealed a hitherto unknown near-oscillatory relaxation of the levitation force with varying magnetic field. This kind of behavior is found to be more pronounced at smaller gap distances between the permanent magnet and the superconductor. A switch-type polarity bistable equilibrium model for the supercurrent structure has been proposed based on the understanding that even the permanent magnet gets magnetized in the presence of the superconductor, which has also been verified and reported here. This model satisfactorily explains the observed oscillatory behavior of relaxation rates. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnetic relaxation phenomena in superconductors have been an intense topic of research over the last decade and a half due to some of the challenges that these phenomena have thrown. In the high Tc oxide superconductors, the magnetic relaxation due to flux creep occurs, resulting in time decay of levitation force. It has been understood that the magnetization relaxes logarithmically [1] in the high Tc materials as
S
dM=dðlntÞ M0
This kind of logarithmic relaxation of magnetization M directly reflects on levitation force as well [1]
S
dF=dðlntÞ K B T ¼ F0 U
where F is the levitation force of the superconductor and U is the height of the pinning energy barrier [2]. The critical current density of oxide superconductor depends on the ability of the material to pin the magnetic flux. Pinning is known to arrest the motion of the flux bundles thereby avoiding dissipation allowing the material to carry large currents up to high magnetic fields. The levitation force measurements can act as a preliminary and important tool to assess the quality of the material. The relaxation of magnetization in these materials can be indirectly extracted by performing measurements of time relaxation of ⇑ Corresponding author. Tel.: +91 40 23134365; fax: +91 40 23130227. E-mail address: [email protected] (V. Seshubai). 0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2011.04.002
levitation force in these materials. By studying the decay of levitation force, one can learn about the vortex dynamics and the associated flux motion [3]. These measurements might also throw light on the nature of pinning forces in the high Tc oxide superconducting materials. From the application point of view, such measurements provide information about the stability of these systems and their suitability for practical applications [4]. For these measurements, we have built a home-made levitation measurement instrument operable at 77 K. Though there are reports on studies of magnetic relaxation through levitation force measurements, to our best knowledge, there is none on time relaxation behavior of levitation force with varying magnetic field in a levitation hysteresis. This work is an attempt to study this behavior in order to understand the relaxation behavior and the underlying vortex dynamics in a fundamental way. 2. Experiment Silver doped YBCO superconductor made by IGP [5] has been used for these measurements. The precursors were made by chemical route and the pellets were processed in air with Nd-123 cold seeding. The processed YBCO/Ag sample was cut and oxygenated at 460 °C for 120 h. Large bulk specimen of thickness 8 mm and area 16 mm 16 mm was used for this experiment coded as SC hereafter. The levitation force relaxation measurements were carried out using a set-up consisting of a container to hold liquid nitrogen and a depth gauge to measure the gap distance. All the measurements were performed at 77 K. A SmCo5 permanent magnet (PM) of 16 mm diameter and 12.5 mm thickness was used. The field
396
R. Parthasarathy et al. / Physica C 471 (2011) 395–399
Fig. 1. Schematic representation of the set-up to measure the relaxation of levitation force. Inset: The inset shows the schematic set-up for measuring the magnetization of the permanent magnet due to the superconductor as the superconductor approaches the magnet from a distance and then recedes.
Fig. 2. Time relaxation of levitation force during the entire hysteresis cycle at different gap distances between the PM and the superconductor. The closed and open symbols correspond to the approaching and receding segments respectively of the magnet and superconductor.
profile of the PM along the z-axis, Bz, was measured using a Hall probe and the field at the center of its surface was found to be 338 mT. Levitation force relaxation was measured up to 120 s at each distance for various distances between the PM and the superconductor as the zero field cooled superconductor approaches the permanent magnet placed on an electronic balance shown in Fig. 1. The measurements were carried out only up to 120 s at each magnetic field value since our IG processed samples have strong flux pinning and we observed that the relaxation is arrested beyond
120 s typically and there is no fall in levitation force beyond this time scale. It has been reported that the force decays logarithmically [2,4,6] with time for time periods of duration similar to those used in our measurements. Our results support these reports as shown in Fig. 2. The levitation force was calculated directly from the change in weight of the PM. Doing so a full hysteresis for the levitation force was obtained at each 30 s time intervals up to 120 s as shown in Fig. 3. The set-up has X–Y motion platform to adjust the magnet position with respect to the superconductor and a vertical digital
R. Parthasarathy et al. / Physica C 471 (2011) 395–399
Fig. 3. Levitation force hysteresis loops at intervals of 30 s up to 120 s.
depth gauge with an accuracy of 0.01 mm for the gap distance measurement.
397
It is interesting to note that there is no monotonous variation of the levitation force decay with time as one would expect particularly at close gap distances between the superconductor and the permanent magnet. As the superconductor is subjected to increasing magnetic fields one might expect the relaxation in magnetization and its rate to change monotonically. But this does not happen and the magnitude of fall in levitation force oscillates with the gap distance (or magnetic field) for a given interval of time both while approaching as well as while receding. The relaxation behavior otherwise follows a well-reported logarithmic decay at each gap distance with time [2] when the magnetic field does not vary. At farther gap distances though, the force decay rate DF/Dt decreases with distance. In this regime, the decay rate DF/Dt could be fitted to an exponential. Beyond a certain distance, relaxation rates are faster at certain gap distances and slower at certain others. Subdued relaxation implies strong and effective pinning. Hence the variation in relaxation rates could imply redistribution of flux lines in the matrix from weak to strong pinning sites and vice versa.
4. Theory and model 3. Results and discussions The relaxation force values were taken at each distance for 120 s at 30 s intervals. It was found that the change in force as well as its rate DF/Dt at various distances was not uniform or monotonous but oscillatory (Fig. 4). The oscillatory behavior was more pronounced at closer distances between the PM and the SC. This rate DF/Dt remains oscillatory during any of these 30 s time intervals. This kind of behavior in SC has not been observed or reported so far to the best of our knowledge.
This kind of behavior could possibly be due to fundamental nature of pinning forces in these materials. The nature of these pinning forces might be such that the velocity of flux motion is nonuniform. We propose that a systematic change in relaxation rate in a near oscillatory manner could be due to switching over of the current structure from unipolar to bipolar [7] or multipolar one [8]. This shift from one type of current structure to another type occurs as the magnetization varies. As the magnetic field varies, re-magnetization of the material occurs resulting in a modified
Fig. 4. (a) Fall in levitation force during various time intervals from the initial time up to 120 s from the initial time. (b) Fall in levitation force per unit time during various time intervals from the initial time up to 120 s from the initial time. (c) Fall in levitation force during time intervals of same duration of 30 s at different times from the initial time. (d) Fall in levitation force during time intervals of same duration of 30 s at different times from the initial time.
398
R. Parthasarathy et al. / Physica C 471 (2011) 395–399
Fig. 5. The left-side part of the figure shows an illustration of unipolar current structure and the right-side part shows that of a bipolar current structure as the remagnetization of the material occurs and the flux structure undergoes changes.
@Bz 1 @ðruBz Þ ¼ r @r @t
Fig. 6. Magnetic hysteresis loops of the permanent magnet magnetized by the superconductor showing its relaxation as well up to 120 s.
flux structure causing a shift from unipolar to bipolar current structure as shown in Fig. 5. This is to say that the system has a bistable equilibrium. During one state the system has a flux structure corresponding to rapid relaxation in magnetization and during the other state the flux structure corresponds and leads to suppressed relaxation [9]. We discuss below as to why this kind of behavior might be possible. According to Anderson and Kim model [10], the current density is related to vortex density gradient caused by pinning as
1 @Bz Jh ¼ ðrÞ l0 @r where Jh is the azimuthal component of current density and r is the radial vector in the cylindrical geometry. The current density Jh changes in sign from positive to negative depending on whether the @Bz/@r is positive or negative [8] as the critical current density Jc is constant throughout the sample volume as per the Bean’s model [11]. That is,
Jh ¼ Jc
if
@Bz < 0 and J h ¼ J c @r
if
@Bz >0 @r
Hence Jh changes sign from positive to negative as the gradient of Bz changes sign. Also, the continuity equation for the density of flux lines demands that,
where u is the flux creep velocity in the material. The gradient of Bz causes a change in Jc. To keep the Jc constant as per Bean’s model, currents of opposite polarities have to be generated and the current structure in the material has to change from being unipolar to bipolar. This can happen in the material when there is a suppression of the magnetic relaxation in the presence of a ferromagnet as a consequence of reversal of polarities [12,13]. It has been suggested in these works that the permanent magnet might also get magnetized to an extent in the presence of the superconductor which in turn induces currents in the superconductor, which circulate in a direction opposite to the original currents. The change in magnetization of the permanent magnet in the presence of the superconductor has been verified by our experiment and reported below in the next section. This kind of a behavior gives rise to a switch-type polarity change in the material thereby resulting in a near-oscillatory behavior of levitation force relaxation rate. Also, it has been reported that the relaxation is suppressed in the presence of a transverse ac magnetic field [14]. In the present work, as the magnet approaches the sample the magnetization increases and hence the reverse magnetization of the PM in the presence of the superconductor also increases. So, there is a competition between the magnetization and the reverse magnetization which is suppression of the original magnetization, resulting in a near-oscillatory behavior. This, we feel, convincingly explains the observed alternate high and low decay rates of levitation force with applied magnetic field. 4.1. Evidence from experiment showing magnetization of the permanent magnet in the presence of the superconductor In order to verify whether the magnetization of the permanent magnet also changes in the presence of the superconductor as suggested by Smolyak and Ermakov [12], we measured the magnetization and its relaxation at the surface of the permanent magnet as the superconductor approaches the magnet surface using a Hall probe fixed to the surface of the permanent magnet as shown in the inset of Fig. 1. Simultaneously we recorded the levitation force of the superconductor and its relaxation as well. This experiment was performed for the full levitation hysteresis loop run. The magnetization loop obtained in this experiment resembles the typical M–H loop of a ferromagnet as shown in Fig. 6. To our best knowledge, this is the first time that a permanent magnet has been magnetized and its M–H loop obtained and
R. Parthasarathy et al. / Physica C 471 (2011) 395–399
reported, using a superconductor. This M–H curve is the typical first quadrant hysteresis curve of a ferromagnet. The magnetic field at the center of the permanent magnet surface was 338 mT in the absence of the superconductor. As the superconductor is cooled below its Tc and made to approach the SmCo5 permanent magnet, the magnetization of the permanent magnet changes in the presence of the superconductor and when the superconductor is withdrawn after approaching the closest possible distance in our set-up, the magnetization of the permanent magnet reverts hysteretically. The magnetization at the surface of the permanent magnet changes from an initial value of 338 mT to a maximum value of 362 mT during the reverse curve of the hysteresis loop. Thus, we clearly see a rise in the magnetization of the permanent magnet in the presence of the superconductor which acts as a corroboration to our bistable equilibrium theory. 5. Conclusions This article presents some important results regarding the magnetic relaxation behavior in superconductors through relaxation of levitation forces. It was observed that in bulk IG processed YBCO/ Ag samples, the levitation force relaxation rates do not vary monotonously as the applied field is increased or decreased, but shows a near-oscillatory behavior which is a surprising and curious result, implying that for certain applied fields the relaxation is faster and at others the relaxation is slower. It is proposed that the observed near-oscillatory behavior might be due to a switch-type
399
polarity and the bistable equilibrium of the current structure in the superconductor in the presence of an external magnetic field due to the change in the magnetization of the permanent magnet by the superconductor providing experimental evidence also to some of the recent studies. Acknowledgements RP thanks UGC for providing RFSMS fellowship under the UGCCAS scheme. MML thanks UGC for Junior Research Fellowship. References [1] Y. Yeshurun, A.P. Malozemoff, A. Shaulov, Rev. Mod. Phys. 68 (1996) 911. [2] Y.S. Tseng, C.H. Chiang, W.C. Chan, Phys. C 411 (2004) 32. [3] G. Blatter, M.V. Feigel’man, V.B. Geshkenbein, A.I. Larkin, Rev. Mod. Phys. 66 (1994) 1125. [4] F.C. Moon, Phys. Lett. 56 (1990) 397. [5] N. Devendra Kumar, T. Rajasekharan, K. Muraleedharan, A. Banerjee, V. Seshubai, Supercond. Sci. Technol. 23 (2010) 105020. [6] Xing-Yi Zhang, Jun Zhou, You-He Zhou, Xin-Wen Liang, Supercond. Sci. Technol. 22 (2009) 025006. [7] B.M. Smolyak, G.N. Perel’shtein, G.V. Ermakov, Tech. Phys. Lett. 27 (2001) 674. [8] B.M. Smolyak, G.N. Perel’shtein, G.V. Ermakov, Cryogenics 42 (2002) 635. [9] M. Chandran, Europhys. Lett. 49 (2000) 494. [10] P.W. Anderson, Y.B. Kim, Rev. Mod. Phys. 36 (1964) 39. [11] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. [12] B.M. Smolyak, G.V. Ermakov, Tech. Phys. Lett. 36 (2010) 461. [13] B.M. Smolyak, G.V. Ermakov, Phys. C 470 (2010) 218. [14] L.M. Fisher, A.V. Kalinov, I.F. Voloshin, V.A. Yampol’skii, Phys. Rev. B 71 (2005) 140503.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Near-oscillatory relaxation behavior of levitation force in infiltration and growth processed bulk YBCO/Ag superconducting composites R. Parthasarathy, M.M. Lakshmi, V. Seshubai ⇑ School of Physics, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e
i n f o
Article history: Received 25 November 2010 Received in revised form 14 March 2011 Accepted 7 April 2011 Available online 13 April 2011 Keywords: Relaxation of levitation force YBCO/Ag superconducting composites Bistable equilibrium model Current structure in superconductor
a b s t r a c t Time relaxation behavior of levitation force has been studied in IGP bulk YBCO/Ag superconductor using levitation force measurements as these measurements throw light on the magnetic relaxation in superconductors and the underlying vortex dynamics, pinning mechanisms and the nature of pinning forces. The measurements have revealed a hitherto unknown near-oscillatory relaxation of the levitation force with varying magnetic field. This kind of behavior is found to be more pronounced at smaller gap distances between the permanent magnet and the superconductor. A switch-type polarity bistable equilibrium model for the supercurrent structure has been proposed based on the understanding that even the permanent magnet gets magnetized in the presence of the superconductor, which has also been verified and reported here. This model satisfactorily explains the observed oscillatory behavior of relaxation rates. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnetic relaxation phenomena in superconductors have been an intense topic of research over the last decade and a half due to some of the challenges that these phenomena have thrown. In the high Tc oxide superconductors, the magnetic relaxation due to flux creep occurs, resulting in time decay of levitation force. It has been understood that the magnetization relaxes logarithmically [1] in the high Tc materials as
S
dM=dðlntÞ M0
This kind of logarithmic relaxation of magnetization M directly reflects on levitation force as well [1]
S
dF=dðlntÞ K B T ¼ F0 U
where F is the levitation force of the superconductor and U is the height of the pinning energy barrier [2]. The critical current density of oxide superconductor depends on the ability of the material to pin the magnetic flux. Pinning is known to arrest the motion of the flux bundles thereby avoiding dissipation allowing the material to carry large currents up to high magnetic fields. The levitation force measurements can act as a preliminary and important tool to assess the quality of the material. The relaxation of magnetization in these materials can be indirectly extracted by performing measurements of time relaxation of ⇑ Corresponding author. Tel.: +91 40 23134365; fax: +91 40 23130227. E-mail address: [email protected] (V. Seshubai). 0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2011.04.002
levitation force in these materials. By studying the decay of levitation force, one can learn about the vortex dynamics and the associated flux motion [3]. These measurements might also throw light on the nature of pinning forces in the high Tc oxide superconducting materials. From the application point of view, such measurements provide information about the stability of these systems and their suitability for practical applications [4]. For these measurements, we have built a home-made levitation measurement instrument operable at 77 K. Though there are reports on studies of magnetic relaxation through levitation force measurements, to our best knowledge, there is none on time relaxation behavior of levitation force with varying magnetic field in a levitation hysteresis. This work is an attempt to study this behavior in order to understand the relaxation behavior and the underlying vortex dynamics in a fundamental way. 2. Experiment Silver doped YBCO superconductor made by IGP [5] has been used for these measurements. The precursors were made by chemical route and the pellets were processed in air with Nd-123 cold seeding. The processed YBCO/Ag sample was cut and oxygenated at 460 °C for 120 h. Large bulk specimen of thickness 8 mm and area 16 mm 16 mm was used for this experiment coded as SC hereafter. The levitation force relaxation measurements were carried out using a set-up consisting of a container to hold liquid nitrogen and a depth gauge to measure the gap distance. All the measurements were performed at 77 K. A SmCo5 permanent magnet (PM) of 16 mm diameter and 12.5 mm thickness was used. The field
396
R. Parthasarathy et al. / Physica C 471 (2011) 395–399
Fig. 1. Schematic representation of the set-up to measure the relaxation of levitation force. Inset: The inset shows the schematic set-up for measuring the magnetization of the permanent magnet due to the superconductor as the superconductor approaches the magnet from a distance and then recedes.
Fig. 2. Time relaxation of levitation force during the entire hysteresis cycle at different gap distances between the PM and the superconductor. The closed and open symbols correspond to the approaching and receding segments respectively of the magnet and superconductor.
profile of the PM along the z-axis, Bz, was measured using a Hall probe and the field at the center of its surface was found to be 338 mT. Levitation force relaxation was measured up to 120 s at each distance for various distances between the PM and the superconductor as the zero field cooled superconductor approaches the permanent magnet placed on an electronic balance shown in Fig. 1. The measurements were carried out only up to 120 s at each magnetic field value since our IG processed samples have strong flux pinning and we observed that the relaxation is arrested beyond
120 s typically and there is no fall in levitation force beyond this time scale. It has been reported that the force decays logarithmically [2,4,6] with time for time periods of duration similar to those used in our measurements. Our results support these reports as shown in Fig. 2. The levitation force was calculated directly from the change in weight of the PM. Doing so a full hysteresis for the levitation force was obtained at each 30 s time intervals up to 120 s as shown in Fig. 3. The set-up has X–Y motion platform to adjust the magnet position with respect to the superconductor and a vertical digital
R. Parthasarathy et al. / Physica C 471 (2011) 395–399
Fig. 3. Levitation force hysteresis loops at intervals of 30 s up to 120 s.
depth gauge with an accuracy of 0.01 mm for the gap distance measurement.
397
It is interesting to note that there is no monotonous variation of the levitation force decay with time as one would expect particularly at close gap distances between the superconductor and the permanent magnet. As the superconductor is subjected to increasing magnetic fields one might expect the relaxation in magnetization and its rate to change monotonically. But this does not happen and the magnitude of fall in levitation force oscillates with the gap distance (or magnetic field) for a given interval of time both while approaching as well as while receding. The relaxation behavior otherwise follows a well-reported logarithmic decay at each gap distance with time [2] when the magnetic field does not vary. At farther gap distances though, the force decay rate DF/Dt decreases with distance. In this regime, the decay rate DF/Dt could be fitted to an exponential. Beyond a certain distance, relaxation rates are faster at certain gap distances and slower at certain others. Subdued relaxation implies strong and effective pinning. Hence the variation in relaxation rates could imply redistribution of flux lines in the matrix from weak to strong pinning sites and vice versa.
4. Theory and model 3. Results and discussions The relaxation force values were taken at each distance for 120 s at 30 s intervals. It was found that the change in force as well as its rate DF/Dt at various distances was not uniform or monotonous but oscillatory (Fig. 4). The oscillatory behavior was more pronounced at closer distances between the PM and the SC. This rate DF/Dt remains oscillatory during any of these 30 s time intervals. This kind of behavior in SC has not been observed or reported so far to the best of our knowledge.
This kind of behavior could possibly be due to fundamental nature of pinning forces in these materials. The nature of these pinning forces might be such that the velocity of flux motion is nonuniform. We propose that a systematic change in relaxation rate in a near oscillatory manner could be due to switching over of the current structure from unipolar to bipolar [7] or multipolar one [8]. This shift from one type of current structure to another type occurs as the magnetization varies. As the magnetic field varies, re-magnetization of the material occurs resulting in a modified
Fig. 4. (a) Fall in levitation force during various time intervals from the initial time up to 120 s from the initial time. (b) Fall in levitation force per unit time during various time intervals from the initial time up to 120 s from the initial time. (c) Fall in levitation force during time intervals of same duration of 30 s at different times from the initial time. (d) Fall in levitation force during time intervals of same duration of 30 s at different times from the initial time.
398
R. Parthasarathy et al. / Physica C 471 (2011) 395–399
Fig. 5. The left-side part of the figure shows an illustration of unipolar current structure and the right-side part shows that of a bipolar current structure as the remagnetization of the material occurs and the flux structure undergoes changes.
@Bz 1 @ðruBz Þ ¼ r @r @t
Fig. 6. Magnetic hysteresis loops of the permanent magnet magnetized by the superconductor showing its relaxation as well up to 120 s.
flux structure causing a shift from unipolar to bipolar current structure as shown in Fig. 5. This is to say that the system has a bistable equilibrium. During one state the system has a flux structure corresponding to rapid relaxation in magnetization and during the other state the flux structure corresponds and leads to suppressed relaxation [9]. We discuss below as to why this kind of behavior might be possible. According to Anderson and Kim model [10], the current density is related to vortex density gradient caused by pinning as
1 @Bz Jh ¼ ðrÞ l0 @r where Jh is the azimuthal component of current density and r is the radial vector in the cylindrical geometry. The current density Jh changes in sign from positive to negative depending on whether the @Bz/@r is positive or negative [8] as the critical current density Jc is constant throughout the sample volume as per the Bean’s model [11]. That is,
Jh ¼ Jc
if
@Bz < 0 and J h ¼ J c @r
if
@Bz >0 @r
Hence Jh changes sign from positive to negative as the gradient of Bz changes sign. Also, the continuity equation for the density of flux lines demands that,
where u is the flux creep velocity in the material. The gradient of Bz causes a change in Jc. To keep the Jc constant as per Bean’s model, currents of opposite polarities have to be generated and the current structure in the material has to change from being unipolar to bipolar. This can happen in the material when there is a suppression of the magnetic relaxation in the presence of a ferromagnet as a consequence of reversal of polarities [12,13]. It has been suggested in these works that the permanent magnet might also get magnetized to an extent in the presence of the superconductor which in turn induces currents in the superconductor, which circulate in a direction opposite to the original currents. The change in magnetization of the permanent magnet in the presence of the superconductor has been verified by our experiment and reported below in the next section. This kind of a behavior gives rise to a switch-type polarity change in the material thereby resulting in a near-oscillatory behavior of levitation force relaxation rate. Also, it has been reported that the relaxation is suppressed in the presence of a transverse ac magnetic field [14]. In the present work, as the magnet approaches the sample the magnetization increases and hence the reverse magnetization of the PM in the presence of the superconductor also increases. So, there is a competition between the magnetization and the reverse magnetization which is suppression of the original magnetization, resulting in a near-oscillatory behavior. This, we feel, convincingly explains the observed alternate high and low decay rates of levitation force with applied magnetic field. 4.1. Evidence from experiment showing magnetization of the permanent magnet in the presence of the superconductor In order to verify whether the magnetization of the permanent magnet also changes in the presence of the superconductor as suggested by Smolyak and Ermakov [12], we measured the magnetization and its relaxation at the surface of the permanent magnet as the superconductor approaches the magnet surface using a Hall probe fixed to the surface of the permanent magnet as shown in the inset of Fig. 1. Simultaneously we recorded the levitation force of the superconductor and its relaxation as well. This experiment was performed for the full levitation hysteresis loop run. The magnetization loop obtained in this experiment resembles the typical M–H loop of a ferromagnet as shown in Fig. 6. To our best knowledge, this is the first time that a permanent magnet has been magnetized and its M–H loop obtained and
R. Parthasarathy et al. / Physica C 471 (2011) 395–399
reported, using a superconductor. This M–H curve is the typical first quadrant hysteresis curve of a ferromagnet. The magnetic field at the center of the permanent magnet surface was 338 mT in the absence of the superconductor. As the superconductor is cooled below its Tc and made to approach the SmCo5 permanent magnet, the magnetization of the permanent magnet changes in the presence of the superconductor and when the superconductor is withdrawn after approaching the closest possible distance in our set-up, the magnetization of the permanent magnet reverts hysteretically. The magnetization at the surface of the permanent magnet changes from an initial value of 338 mT to a maximum value of 362 mT during the reverse curve of the hysteresis loop. Thus, we clearly see a rise in the magnetization of the permanent magnet in the presence of the superconductor which acts as a corroboration to our bistable equilibrium theory. 5. Conclusions This article presents some important results regarding the magnetic relaxation behavior in superconductors through relaxation of levitation forces. It was observed that in bulk IG processed YBCO/ Ag samples, the levitation force relaxation rates do not vary monotonously as the applied field is increased or decreased, but shows a near-oscillatory behavior which is a surprising and curious result, implying that for certain applied fields the relaxation is faster and at others the relaxation is slower. It is proposed that the observed near-oscillatory behavior might be due to a switch-type
399
polarity and the bistable equilibrium of the current structure in the superconductor in the presence of an external magnetic field due to the change in the magnetization of the permanent magnet by the superconductor providing experimental evidence also to some of the recent studies. Acknowledgements RP thanks UGC for providing RFSMS fellowship under the UGCCAS scheme. MML thanks UGC for Junior Research Fellowship. References [1] Y. Yeshurun, A.P. Malozemoff, A. Shaulov, Rev. Mod. Phys. 68 (1996) 911. [2] Y.S. Tseng, C.H. Chiang, W.C. Chan, Phys. C 411 (2004) 32. [3] G. Blatter, M.V. Feigel’man, V.B. Geshkenbein, A.I. Larkin, Rev. Mod. Phys. 66 (1994) 1125. [4] F.C. Moon, Phys. Lett. 56 (1990) 397. [5] N. Devendra Kumar, T. Rajasekharan, K. Muraleedharan, A. Banerjee, V. Seshubai, Supercond. Sci. Technol. 23 (2010) 105020. [6] Xing-Yi Zhang, Jun Zhou, You-He Zhou, Xin-Wen Liang, Supercond. Sci. Technol. 22 (2009) 025006. [7] B.M. Smolyak, G.N. Perel’shtein, G.V. Ermakov, Tech. Phys. Lett. 27 (2001) 674. [8] B.M. Smolyak, G.N. Perel’shtein, G.V. Ermakov, Cryogenics 42 (2002) 635. [9] M. Chandran, Europhys. Lett. 49 (2000) 494. [10] P.W. Anderson, Y.B. Kim, Rev. Mod. Phys. 36 (1964) 39. [11] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. [12] B.M. Smolyak, G.V. Ermakov, Tech. Phys. Lett. 36 (2010) 461. [13] B.M. Smolyak, G.V. Ermakov, Phys. C 470 (2010) 218. [14] L.M. Fisher, A.V. Kalinov, I.F. Voloshin, V.A. Yampol’skii, Phys. Rev. B 71 (2005) 140503.